NEW PROCESS REQUIREMENTS FOR ADDITIVE POWDERS FOR MICROPLASMА POWDER DEPOSITION 

Abstract

The distribution of doping elements and impurities between the external surfaces and internal volumes of typical fine particles in samples of gas-atomized commercial additive powders of high-temperature creep-resistant (Inconel 939, ZhS32) and high-temperature oxidation-resistant (Inconel 625, Hastelloy C22) nickel-based superalloys was examined employing energy-dispersive X-ray (EDX) analysis. Significant concentration gradients were observed between the surfaces and internal volumes of powder particles for doping elements with a content of up to 4–5 wt.%: Re, Mo Ta, and Nb in high-temperature creep-resistant alloys and Al, Nb, Co, Fe, V, and Mn in the Inconel 625 high-temperature oxidation-resistant alloy. Besides doping elements, concentration gradients of O, N, S, P, and Si impurities were found in the near-surface layers of the additive powders. The EDX findings and data from the reduction–extraction method were used to calculate the amounts of oxygen and nitrogen in the internal volumes and the near-surface layer of typical fine powder particles and the thickness of this layer corresponding to the increased content of these impurities. The surface layer of typical fine particles was shown to increase the total weight-average content of impurities in the samples of commercial additive powders: oxygen up to 2.5 times and nitrogen up to 1.8 times. To assess the influence of impurity amounts of oxygen at <0.16 wt.% and nitrogen at <0.13 wt.% on the welding process properties of atomized additive powders, additional samples of high-temperature oxidation-resistant (ChS40) and high-temperature creep-resistant (ZhS6U, ZhS32, Renè 80) nickel superalloys were tested to ascertain their suitability for microplasma powder deposition at a welding current of up to 15 A. The additive powder was found to be suitable for this low-amperage deposition mainly because of its limited oxygen impurity content: weight-average content up to 0.025 wt.% and content in the surface layer of a typical fine particle 1–3 μm thick up to 0.1 wt.%.